June 27, 2005
10:00 AM (EDT)

News Release Number: STScI-2005-16

Hubble Captures Outburst from Comet Targeted By Deep Impact

June 27, 2005: In a dress rehearsal for the rendezvous between NASA's Deep Impact
spacecraft and comet 9P/Tempel 1, the Hubble Space Telescope captured
dramatic images of a new jet of dust and gas streaming from the icy
comet. The images are a reminder that Tempel 1's icy nucleus, roughly
half the size of Manhattan, is dynamic and volatile. Astronomers hope
the eruption of dust seen in these observations is a preview of the
fireworks that may come July 4, when a probe from the Deep Impact
spacecraft slams into the comet, possibly blasting off material and
giving rise to a similar plume of dust and gas.

Q & A: Understanding the Discovery

1.
Why do comets have jets?

Comets are thought to be "dirty snowballs," porous collections of ice
and rock that dwell in the frigid outer boundaries of our solar system.
Periodically, they make their journey into the inner solar system as
they loop around the Sun. As a comet approaches the Sun, sunlight heats
up its icy nucleus. Sometimes the extreme heat opens cracks in the
comet's dark, crusty surface. Dust and gas trapped beneath the surface
could then spew out of the crack, forming a jet. Sometimes a portion of
the crust itself is lifted off the nucleus by the pressure of heated
gases beneath the surface. This porous crust might then crumble into
small dust particles shortly after leaving the nucleus, producing a
fan-shaped coma on the sunward side. Ultimately, for comets bigger,
brighter, and closer to the Sun than Tempel 1, the gas and dust that is
ejected in jets or sublimated from the comet's surface stretches out in
orbit behind the comet, forming its familiar, beautiful tail.

2.
Why are the Hubble images dark on one side and bright on the other?

Due to its rotation, irregular shape, and uneven surface composition,
the comet's solid nucleus is ejecting gas and dust at a variable rate.
So the distribution of material around the comet is also irregular –
denser in some places and less dense in others. The darkest wedge on the
side opposite the jet is the shadow cast by the inner coma and nucleus.

3.
Will the outburst alter the planned July 4 encounter between the Deep Impact spacecraft and Tempel 1?

Astronomers believe that this jet, if it lasts that long, will be on the
side of the comet away from the Deep Impact spacecraft at the time of
the encounter. So, no changes are planned in the flight path toward the
comet.

4.
The Deep Impact spacecraft has cameras that will be taking close-up images of the impact, so why will astronomers use the Hubble telescope to snap views of the collision?

The observations by the Deep Impact camera and the Hubble telescope
complement each other. The camera aboard the Deep Impact spacecraft will
provide a brief, close-up view of the comet, from more than 300 miles
away. The Hubble telescope, from its distance 80 million miles away,
will capture a broader view of the encounter. Hubble also will continue
to monitor the comet's activities for several days after the event.

5.
What do astronomers expect the Hubble telescope to see during the impact?

Hubble will study the expected plume of dust and gas that may be ejected
from the surface of the comet and from below its surface as a result of
the impact. By analyzing the Hubble observations, astronomers will
determine the speed of the ejected material, how far the plume extends
into space, and something about its chemical makeup. Hubble's
observations will include images taken in visible light, as well as
images and spectra taken in ultraviolet light. Some of the gases
expected to be released, such as carbon monoxide, carbon, and sulfur,
can best be seen in ultraviolet light. These gases come from ices that
condensed out of the cloud of material from which the solar system
formed.